1. The Field of the Invention
The present invention relates generally to spin-valve sensors for reading data from a magnetic media and, more particularly to novel structures and processes of spin-valve sensors, and to magnetic recording systems which incorporate such spin-valve sensors
2. The Relevant Art
Computer systems generally utilize auxiliary memory storage devices having magnetic media on which data can be written and from which data can be read for later uses. A direct access storage device, such as a disk drive, incorporating rotating magnetic disks, is commonly used for storing data in a magnetic form on the disk surfaces. Data are written on concentric, radially spaced tracks on the disk surfaces. Magnetic read/write heads are then used to read data from the tracks on the disk surfaces.
FIG. 1 shows one example of a disk drive 100 embodying the present invention. As shown in FIG. 1, the disk drive 100 comprises at least one rotatable magnetic disk 112 supported on a spindle 114 and rotated by a disk drive motor 118. The magnetic medium on each magnetic disk 112 is in the form of concentric, annular data tracks (not shown).
At least one slider 113 is positioned on the disk 112. Each slider 113 supports one or more magnetic read/write heads 121 incorporating one or more read sensors of the present invention. As the magnetic disk rotates, the slider 113 is moved radially in and out over the disk surface 122 so that the magnetic read/write heads 121 may access different portions of the magnetic disk 112 where desired data are written. Each slider 113 is attached to an actuator arm 119 by means of a suspension 115. The suspension 115 provides a slight spring force which biases the slider 113 against the disk surface 122. Each actuator arm 119 is attached to an actuator 127. The actuator 127 as shown in FIG. 1 may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, and the direction and speed of the coil movements are controlled by the motor current signals supplied by a controller 129.
During operation of the disk storage system, the rotation of the magnetic disk 112 generates an air bearing between the surface of the slider 113 (which includes the surface of the head 121) referred to as an air bearing surface (ABS), and the surface 122 of the disk 112. This air bearing exerts an upward force or lift on the slider 113, and thus counter-balances the slight spring force of the suspension 115 and supports the slider 113 off and slightly above the disk surface by a small, substantially constant spacing during normal operation.
The various components of the disk storage system are controlled in operation by control signals generated by the control unit 129. The control signals include access control signals and internal clock signals. Typically, the control unit 129 comprises logic control circuits, storage means, and a microprocessor. The control unit 129 generates control signals to control various system operations such as drive motor control signals on a line 123 and head position and seek control signals on a line 128. The control signals on the line 128 provide the desired current profiles to optimally move and position the slider 113 to the desired data track on the disk 112. Read and write signals are communicated to and from the read/write heads 121 by means of a recording channel 125. In the depicted embodiment, the read/write heads 121 incorporate the read sensor of the present invention.
Two types of read sensors have been extensively explored for magnetic recording at ultrahigh densities (xe2x89xa720 Gb/in2). One is a current-in-plane (CIP) spin-valve sensor 200 in which a sense current 218 flows in a direction parallel to interfaces of a plurality of films, as depicted in FIG. 2. The other is a current-perpendicular-to-plane (CPP) magnetic-tunnel-junction sensor 300 in which a sense current 318 flows in a direction perpendicular to the interfaces of a plurality of films. Greater details will be given to the CPP read sensor of the present invention below with reference to FIG. 3.
In high capacity disk drives, a giant magnetoresistance (GMR) head carrying the CIP spin-valve sensor is now extensively used to read written data from the tracks on the disk surfaces. This CIP spin-valve sensor typically comprises two ferromagnetic films separated by an electrically conducting nonmagnetic film. Due to a GMR effect, the resistance of this CIP spin-valve sensor varies as a function of the spin-dependent transmission of conduction electrons between the two ferromagnetic films and the accompanying spin-dependent scattering which takes place at interfaces of the ferromagnetic and nonmagnetic films.
In this CIP spin-valve sensor, one of the ferromagnetic films, referred to as a transverse pinned layer, typically has its magnetization pinned by exchange coupling with an antiferromagnetic film (e.g., Nixe2x80x94Mn, Ptxe2x80x94Mn, Irxe2x80x94Mn, etc.) used as a transverse pinning layer. The magnetization of the other ferromagnetic film, referred to as a sense or xe2x80x9cfreexe2x80x9d layer, however, is not fixed and is free to rotate in response to the signal field from written data on the magnetic medium. In this CIP spin-valve sensor, the GMR effect varies as the cosine of the angle between the magnetizations of the sense and transverse pinned layers. The written data can be read from the magnetic medium because the external magnetic field from the written data causes a change in the direction of magnetization in of the sense layer, which in turn causes a change in the resistance of the CIP spin-valve sensor and a corresponding change in the sensed current or voltage. It should be noted that an anisotropy magnetoresistance (AMR) effect is also present in the sense layer and tends to reduce the overall GMR effect.
The CIP spin-valve sensor 200 is formed with deposition methods, such as DC magnetron sputtering, ion beam sputtering, etc, onto a wafer and is confined in a central region with two end regions (not shown) that abut the edges of the central region. Seed layers 202 are deposited on the wafer. These seed layers have a face-centered-cubic crystalline structure, which orients the crystalline structures of subsequently deposited films so that the closest packed planes of these films are parallel to the wafer surface. These closest packed planes are believed to play a crucial role in improving GMR properties of the CIP spin-valve sensor 200.
A transverse pinning layer made of an antiferromagnetic film 204 is deposited above the seed layer 202. A keeper layer made of a ferromagnetic film 206 is separated from a reference layer also made of a ferromagnetic film 210 by a ruthenium (Ru) spacer layer 208. The magnetizations of the keeper layer 206 and the reference layer 210 (both of which are used as transverse pinned layers) are fixed through antiferromagnetic/ferromagnetic coupling between the transverse pinning layer 204 and the keeper layer 206, and through ferromagnetic/ferromagnetic coupling across the Ru spacer layer 208. The reference layer 210 is separated by a copper (Cu) spacer layer 212 from a sense layer also made of a ferromagnetic film 214. The cap layer 216 is deposited above the sense layer 214.
The other, more recently explored CPP magnetic-tunnel-junction sensor is shown in FIG. 3. This CPP magnetic-tunnel-junction sensor 300 has a similar sensor structure as that of the CIP spin-valve sensor 200. The primary difference between the two sensors is that the Cu spacer layer used in the CIP spin-valve sensor 200 is replaced by an Alxe2x80x94O barrier layer in the magnetic-tunnel-junction sensor 300.
The disk drive industry has been engaged in an ongoing effort to increase the recording density of the disk drive, and correspondingly to increase the overall signal sensitivity to permit the currently used CIP spin-valve sensor in the disk drive to read smaller changes in magnetic fields. The major property relevant to the signal sensitivity of the CIP spin-valve sensor is its GMR coefficient. A higher GMR coefficient leads to higher signal sensitivity and enables the storage of more data in a unit area on a disk surface. The GMR coefficient of the CIP spin-valve sensor is expressed as xcex94RG/R∥, where R∥ is a resistance measured when magnetizations of the sense and reference layers are parallel to each other, and xcex94RG is the maximum giant magnetoresistance measured when magnetizations of the sense and reference layers are antiparallel to each other.
An additional property relevant to the performance of the CIP spin-valve sensor is ferromagnetic/ferromagnetic coupling between the reference and sense layers. This ferromagnetic/ferromagnetic coupling induces a ferromagnetic coupling field (HF), which must be very well controlled for optimal sensor operation.
In order to achieve higher recording densities, the disk drive industry is constantly miniaturizing the CIP spin-valve sensor. Several challenges have arisen due to the miniaturization of the CIP spin-valve sensor. One area of difficulty has been finding an ideal insulating gap material for the use as thin top and bottom gap layers. To attain, for example, a 38.8 nm thick CIP spin-valve sensor with the sense layer located in the center of a 80 nm thick read gap, the top and bottom gap layers must have thicknesses of 34.4 and 6.8 nm, respectively. A 6.8 nm thick bottom gap layer is too thin to prevent electrical shorting between the bottom magnetic shield layer and the CIP spin-valve sensor. Consequently, there is a high possibility of electrical shorting, making the CIP spin-valve sensor non-functional.
The CPP magnetic-tunnel-junction sensor has been used to solve these issues. The CPP magnetic-tunnel-junction sensor is made of at least two ferromagnetic films separated by an insulating barrier layer. The tunnel magnetoresistance (TMR) coefficient is defined as xcex94RT/R∥, where R∥ is the resistance measured when the magnetizations of the two ferromagnetic films are parallel to each other, and xcex94RT is the maximum tunnel magnetoresistance measured when the magnetizations of the two ferromagnetic films are antiparallel to each other. Since the sense current must flow from a top magnetic shield layer, through the sensor, to a bottom magnetic shield layer, or vice versa, both top and bottom gap layers must be formed of conducting films. As a result, electrical shorting between the top and bottom magnetic shield layers and the sensor, and between the sensor and the top magnetic shield layer, is no longer a concern, and further decreasing of the read gap thickness to below 60 nm becomes feasible.
Issues are also encountered when attempting to use the CPP magnetic-tunnel-junction sensor to increase magnetic recording densities. These issues originate mainly from difficulties in attaining a high TMR coefficient and a low junction resistance simultaneously. For instance, in a typical oxidation process used for the CPP magnetic-tunnel-junction random access memory, where a 1.2 nm thick Al film is exposed for 1 hour in air, a TMR coefficient of 29.3% and a junction resistance of 5714 xcexa9-xcexcm2 are attained. This junction resistance is much higher than a most preferred junction resistance of 0.4 xcexa9-xcexcm2. With this most preferred junction resistance, a CPP magnetic-tunnel-junction sensor with a width of 0.1 xcexcm and a height of 0.1 xcexcm will exhibit an optimal sensor resistance of 40 xcexa9. With the optimal resistance of 40 xcexa9, high signal amplitudes can be attained without concerns on electrostatic discharge.
To substantially reduce the junction resistance to a value in a preferred range of between 0.1 and 10 xcexa9-xcexcm2, an in-situ oxidation process, where a 0.54 nm thick Al film is exposed for 4 min in an oxygen gas of 2 Torr, is applied. After this in-situ oxidation process, a TMR coefficient of 18.5% and a junction resistance of 8 xcexa9-xcexcm2 are attained. Hence, the TMR coefficient and junction resistance of the CPP magnetic-tunnel-junction sensor substantially depend on the thickness, oxidation pressure, and oxidation time of the Alxe2x80x94O barrier layer. As seen from the above discussion, the state-of-the-art CPP magnetic-tunnel-junction sensor is still not viable for the use for magnetic recording.
The difficulty in attaining a low junction resistance originates from the high electrical resistivity of the Alxe2x80x94O film (xe2x89xa7108 xcexcxcexa9-cm). Hence, to attain a low junction resistance, a barrier layer with a low electrical resistivity must be selected. A CIP spin-valve sensor with a Cu spacer layer may be implemented into the CPP magnetic-tunnel-junction sensor structure, and used as a CPP spin-valve sensor. A GMR effects, instead of the tunneling effects, will occur in the CPP spin-valve sensor. Its GMR coefficient is typically higher by approximately 40% than that of the CIP spin-valve sensor. However, this CPP spin-valve sensor with the Cu barrier layer is also not viable due to a low electrical resistivity of the Cu film (xcx9c3 xcexcxcexa9-cm), which will lead to a junction resistance of as low as below 0.001 xcexa9-xcexcm2.
Thus, it can be seen from the above discussion that there is a need existing in the art for an improved CPP spin-valve sensor exhibiting a high GMR coefficient and a low junction resistance simultaneously. Particularly, it would be advantageous to provide a CPP spin-valve sensor exhibiting a junction resistance controlled to be much higher than that of a CPP spin-valve sensor with a Cu barrier layer, but significantly lower than that of a CPP magnetic-tunnel-junction sensor with an Alxe2x80x94O barrier layer.
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available current-in-plane (CIP) spin-valve sensors. Accordingly, it is an overall object of the present invention to provide a current-perpendicular-to-plane (CPP) spin-valve sensor that overcomes many or all of the above-discussed shortcomings in the art.
To achieve the foregoing object, and in accordance with the invention as embodied and broadly described herein in the preferred embodiments, novel CPP spin-valve sensor is provided. The CPP spin-valve sensor of the present invention in one embodiment comprises a metallic oxide barrier layer interposed between the sense layer and the reference layer. Under a preferred embodiment of the present invention, the metallic oxide barrier layer is formed substantially of a Cuxe2x80x94O film with an oxygen content in the range of between about 12 and about 24 at %, with a thickness in the range of between about 2 and about 6 nm, and with an electrical resistivity in the range of between about 100 and about 1600 xcexcxcexa9-cm. The CPP spin-valve sensor also comprises a reference layer formed of a second ferromagnetic film disposed to one side of the sense layer, a keeper layer disposed to one side of the reference layer, a transverse pinning layer disposed to one side of the keeper layer, and longitudinal pinned and pinning layers disposed to another side of the sense layer.
In an alternative embodiment, the CPP spin-valve sensor comprises a plurality of alternating metallic oxide barrier and sense layers. In one embodiment the plurality of alternating metallic oxide barrier and sense layers comprises 3 metallic oxide barrier and 3 sense layers.
The CPP spin-valve sensor of the present invention may be incorporated within a disk drive system comprising a magnetic disk, a CPP spin-valve sensor configured in the manner discussed above, an actuator for moving the CPP spin-valve sensor across the magnetic disk so that the CPP spin-valve sensor may access different regions of written data on the magnetic disk, and a detector. The detector may be electrically coupled to the CPP spin-valve sensor for detecting changes in resistance of the sensor caused by the rotation of the magnetization of the sense layer relative to the fixed net magnetizations of the reference and keeper layers in response to magnetic fields from the written data.
A method of fabrication of the present invention is also presented for forming a metallic oxide barrier layer of a CPP spin-valve sensor. In one embodiment the fabrication method comprises forming the afore-mentioned layers of the CPP spin-valve sensor, and forming a metallic oxide barrier layer to one side of the reference layer by depositing a metallic film using reactive DC-pulsed sputtering in a first mixture of argon and oxygen gases and subsequent in-situ oxidizing in a second mixture of argon and oxygen gases.
The thickness of the oxygen-doped/in-situ oxidized film is in the range of between about 2 nm and about 6 nm. In one embodiment, the oxygen doping process is conducted preferably in the first mixture of argon and oxygen gases of 2.985 and 0.015 mTorr, respectively. The in-situ oxidation process is conducted preferably in the second mixture of argon and oxygen gases of 2.94 and 0.06 mTorr, respectively.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.